Most of today's automobiles are equipped with closed-loop, electronic controls supported by on-board microcomputers so as to perform a variety of control functions. Thus, for example, electronic controls are provided to optimize fuel economy and engine operation, meet emission control requirements and to provide for more comfortable and/or safe driving characteristics for the automobile (e.g., such as those characteristics provided by antilocking and/or antiskid braking systems, positive traction systems, suspension adjustment systems and the like).
All of these automotive control systems are dependant upon the ability of the electronic control loop to sense accurately the operating variable(s) of the automobile system under control, and then to exhibit the desired rate-responsiveness in order to exercise adequate control. As more sophisticated electronic control schemes have evolved, it is the sensors which have become performance limiting factors due principally to the inability of sensor fabrication technology to keep pace with the development of integrated automobile control systems.
Recently, however, "micromachining" techniques for forming structural three-dimensional devices from silicon have emerged as a cost-effective means of producing high quality (i.e., sufficiently sensitive) durable sensors useful for the automotive industry. (See, Lee et al, "Silicon Micromachining Technology for Automotive Applications", SAE Publication No. SP655, February, 1986, the entire content of which is expressly incorporated hereinto by reference.)
By way of the present invention, novel micromachining techniques are employed to fabricate equally novel forms of sensors useful in closed loop, electronic controls for automobiles.
According to the invention, silicon-based sensors and methods of sensor fabrication employing silicon micromachining techniques are provided. The sensors of the invention generally include a substrate, a sensor element, and a protective diaphragm mounting and covering the sensor element to the substrate. The diaphragm encompasses, and therefore protects, the sensor from its environment. The protective diaphragm of this invention is one which is formed of an etch-stop doped silicon layer sealed to the substrate.
During fabrication, the etch-stop doped layer is formed on the surface of a recessed "trough" in a block of silicon (which silicon block may be a chip or wafer of the type conventionally used in integrated circuit fabrication technology). The sensor element is then formed over the etch-stop doped layer in the trough (as by removing unnecessary regions of a previously applied metallized layer), and the doped layer is then sealed to the surface of a substrate (e.g., preferably a glass substrate having similar thermal expansion properties to that of silicon). The sensor element is thus "sandwiched" between the doped layer and the substrate. The sensor element is preferably spaced from the substrate particularly if this invention is embodied in the form of a mass air flow sensor.
Undoped regions of the silicon block may then be etched away leaving the etch-stop doped layer as a protective diaphragm covering and mounting the sensor element to the substrate. That is, if the substrate to which the doped silicon layer is sealed is considered to be the front of the sensor, then the etching of undoped silicon regions is accomplished from the back of the sensor (i.e., "back etching"). In this manner, the sensor element is encapsulated by means of the substrate and its protective etch-stop doped silicon diaphragm.
The general techniques described above may also be employed to expose a metallized bonding pad(s) or the like so as to facilitate sensor interconnection with electronic circuitry of the control loop in which the sensor is employed. That is, by discontinuously doping a region of a recessed trough in the silicon block with an etch-stop dopant--that is, so that an undoped region of the trough exists ( between, and/or is defined by, the remaining doped trough region(s)--and then forming the bonding pad(s) over such discontinuously doped region(s) in the trough, the bonding pad may be exposed by back-etching away the undoped silicon region(s) in the trough.
This back etching technique of the invention is thus beneficial in that the sensor element may be formed in one area of a trough in a silicon block (in which the trough surface has been continuously doped with an etch-stop dopant) while a bonding pad may be formed in another area in the same (or different) trough in the silicon block (in which the trough surface has been discontinuously doped with an etch-stop dopant). Upon etching away the undoped silicon regions in the block, therefore, the protective diaphragm (which is comprised of an etch-stop doped silicon layer) covers and mounts the sensor element to the encapsulating substrate while a "window" in the etch-stop doped layer is simultaneously formed exposing the bonding pad(s).
These as well as other objects and advantages of this invention will become more clear to the reader after careful consideration is given to the detailed description of the preferred exemplary embodiments thereof which follow.